Women who are obese have nearly double the risk of developing colorectal cancer at a young age than women with a normal body mass index, a recent study by investigators at Dana-Farber and other institutions has found.

The study, prompted by concerns over recent increases in colorectal cancer rates in people under 50, is the first to prospectively examine the possible role of obesity in this trend. Published in JAMA Oncology, the paper calls for more research into the possible biological mechanisms between obesity and early-onset colorectal cancer.

“We know that diet and lifestyle and factors like obesity are associated with the risk of colorectal cancer regardless of age,” says study co-lead author Kimmie Ng, MD, MPH, of Dana-Farber. “And because increases in obesity have paralleled the rise in colorectal cancer rates in individuals under the age of 50, we wondered whether obesity might be playing a role in increasing the risk of developing this cancer in younger people.”

The study drew on data from the Nurses’ Health Study II, which, beginning in 1989, has tracked the health of tens of thousands of female nurses with biennial questionnaires about their diet, lifestyle, and weight. Of 85,256 women who were free of cancer and inflammatory bowel disease when they enrolled in the study, 114 were diagnosed with colorectal cancer over a 14-year period. Their average age was 45.

“We found that the risk of developing colon cancer at such a young age was significantly higher for women categorized as obese based on body mass index, versus those who were not obese or simply overweight,” Ng says. “We also found that being overweight in early adulthood, as well as gaining weight between age 18 and the diagnosis of colon cancer, seems be associated with a higher risk of developing colon cancer at an early age.”

Ng notes that the findings do not show that obesity has a causative effect on early-onset colon cancer, but the findings suggest an association between them. “People who are obese tend to have other dietary and lifestyle factors that also increase the risk of colon cancer, such as diabetes, higher rates of smoking, and lower levels of physical activity,” Ng remarks. “These factors could be contributing to the risk, but the true underlying cause or causes are completely unknown at this point.

“More research is needed to elucidate what the exact risk factor or factors are, but maintaining a healthy weight is beneficial for a variety of reasons,” she continues.

Research should also explore whether the same association between obesity and early-onset colon cancer exists in males.

This article was originally published on April 24, 2019, by Dana-Farber Cancer Institute. It is republished with permission.